IEEE 802.11p protocol, which is also called Wireless Access in Vehicular Environment (WAVE), is an approved amendment to the IEEE 802.11. It is an extension of IEEE 802.11 and conforms to corresponding applications in Intelligent Transportation System (ITS).
Similar with other 802.11 family members, frame synchronization of 802.11p mainly depends on short and long training sequences. An example of such training sequences is illustrated in FIG. 1. In this example, there are ten short training sequences t1˜t10 and two long training sequences T1˜T2, and the total training time is 32 μs.
In some conventional methods, frame synchronization is divided into a coarse timing process and a fine timing process. In the coarse timing process, the position of a data frame is approximately located using a first pre-set threshold and a first timing metric function based on self-correlation of the received signal. In the fine timing process, the position of the data frame is more accurately located using a second pre-set threshold and a second timing metric function based on cross-correlation between the received signal and the known training sequences. A data frame is a data packet containing certain training sequences and payload.
In such methods, the second timing metric function based on cross-correlation is used only when a data frame is detected. Since computation based on self-correlation has low complexity and relatively low precision, and computation based on cross-correlation has relatively high complexity and high precision, relatively high precision at relatively low computation complexity can be achieved by such combination.
In conventional methods, a timing metric function used in fine timing is usually defined as Equation (1),
                              M          ⁡                      (            d            )                          =                  |                                    ∑                              k                =                0                                                              N                  ⁢                                      /                                    ⁢                  2                                -                1                                      ⁢                                                  ⁢                                                            r                  ⁡                                      (                                          k                      +                      d                                        )                                                  ·                s                            *                              (                k                )                                              |                                    Equation        ⁢                                  ⁢                  (          1          )                    where r(d) stands for a received signal, d stands for a time point where the received signal is sampled, s*(k) stands for conjugation of s(k), s(k) stands for the ten short training sequences known to the transmitter and the receiver, and N/2 stands for length of a short training sequence.
A threshold Cth may be pre-set. If there are 10 successive values of the timing metric function within a time period identified by the coarse timing process, which values are evenly spaced by the length of a short training sequence and greater than Cth, it is deemed that the 10 short training sequences are located, and the time point corresponding to the first of the 10 values is taken as the beginning of the 10 short training sequences. An example of how a data frame is located is schematically shown in FIG. 2.
However, related parameters of a signal channel may change such as due to different kinds of fading. As a result, a pre-set threshold may cause errors in certain cases. For example, in some cases, the pre-set threshold may be too low, and noise may be identified as a data frame by mistake. In some cases, the pre-set threshold may be too high, and the beginning part of a frame may be missed. Therefore, more robust fine timing methods and systems are needed.